'Free will offering' in the Bible

and thou hast made the feast of weeks to Jehovah thy God, a tribute of a free-will offering of thy hand, which thou dost give, as Jehovah thy God doth bless thee.

The produce of thy lips thou dost keep, and hast done it, as thou hast vowed to Jehovah thy God; a free-will-offering, which thou hast spoken with thy mouth.